Most of the sniper rifles employ a manual bolt. A shooter manually operates the bolt when a bullet is loaded into a powder chamber of the barrel. The operation of the manual bolt is automated using pressure of the compressed gas, so that the shooter's loading effort of the assembly is lessened. One of the self-loading bolt assemblies for an airgun is disclosed in U.S. Pat. No. 3,572,310 (which is issued on Mar. 23, 1971 to Mr. Chiba). As shown in FIG. 5 of the drawings in this patent, a defect in the conventional self-loading bolt assembly for the airgun resides in its complicated construction. The conventional assembly disadvantageously requires many components, and also requires a “strong firing spring 34” as disclosed in line 36 on column 3 of the patent. Due to the existence of this strong firing spring, when the bullet is fired, a strong reaction follows. This is another defect of the conventional assembly for the airgun.